Migrate compiler, idea and others to new case conversion api

compiler/android-tests/tests/org/jetbrains/kotlin/android/tests/CodegenTestsOnAndroidGenerator.kt

@@ -262,7 +262,7 @@ class CodegenTestsOnAndroidGenerator private constructor(private val pathManager
 
         private fun getFlavorUnitTestFolder(flavourName: String): String {
             return pathManager.srcFolderInAndroidTmpFolder +
-                    "/androidTest${flavourName.capitalize()}/java/" +
+                    "/androidTest${flavourName.replaceFirstChar(Char::uppercaseChar)}/java/" +
                     testClassPackage.replace(".", "/") + "/"
         }
 
@@ -379,7 +379,7 @@ class CodegenTestsOnAndroidGenerator private constructor(private val pathManager
             configure(backend)
             testInfo = KotlinTestInfo(
                 "org.jetbrains.kotlin.android.tests.AndroidRunner",
-                "test${testDataFile.nameWithoutExtension.capitalize()}",
+                "test${testDataFile.nameWithoutExtension.replaceFirstChar(Char::uppercaseChar)}",
                 emptySet()
             )
         }.build(testDataFile.path)

compiler/android-tests/tests/org/jetbrains/kotlin/android/tests/UnitTestFileWriter.kt

@@ -27,7 +27,7 @@ class UnitTestFileWriter(
     }
 
     fun generate() {
-        FileWriter(File(flavourFolder, flavourName.capitalize() + ".java").also { it.parentFile.mkdirs() }).use { suite ->
+        FileWriter(File(flavourFolder, flavourName.replaceFirstChar(Char::uppercaseChar) + ".java").also { it.parentFile.mkdirs() }).use { suite ->
             val p = Printer(suite)
             p.println(
                 """package ${CodegenTestsOnAndroidGenerator.testClassPackage};
@@ -35,7 +35,7 @@ class UnitTestFileWriter(
                 |import ${CodegenTestsOnAndroidGenerator.baseTestClassPackage}.${CodegenTestsOnAndroidGenerator.baseTestClassName};
                 |
                 |/* This class is generated by ${CodegenTestsOnAndroidGenerator.generatorName}. DO NOT MODIFY MANUALLY */
-                |public class ${flavourName.capitalize()} extends ${CodegenTestsOnAndroidGenerator.baseTestClassName} {
+                |public class ${flavourName.replaceFirstChar(Char::uppercaseChar)} extends ${CodegenTestsOnAndroidGenerator.baseTestClassName} {
                 |
             """.trimMargin()
             )

compiler/backend/src/org/jetbrains/kotlin/codegen/coroutines/ChangeBoxingMethodTransformer.kt

@@ -28,7 +28,7 @@ object ChangeBoxingMethodTransformer : MethodTransformer() {
         val map = hashMapOf<String, String>()
         for (primitiveType in JvmPrimitiveType.values()) {
             val name = primitiveType.wrapperFqName.topLevelClassInternalName()
-            map[name] = "box${primitiveType.javaKeywordName.capitalize(Locale.US)}"
+            map[name] = "box${primitiveType.javaKeywordName.replaceFirstChar(Char::uppercaseChar)}"
         }
         wrapperToInternalBoxing = map
     }

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/Properties.kt

@@ -63,9 +63,9 @@ enum class CompilerSystemProperties(val property: String) {
 }
 
 val isWindows: Boolean
-    get() = CompilerSystemProperties.OS_NAME.value!!.toLowerCase(Locale.ENGLISH).startsWith("windows")
+    get() = CompilerSystemProperties.OS_NAME.value!!.lowercase().startsWith("windows")
 
-fun String?.toBooleanLenient(): Boolean? = when (this?.toLowerCase()) {
+fun String?.toBooleanLenient(): Boolean? = when (this?.lowercase()) {
     null -> false
     in listOf("", "yes", "true", "on", "y") -> true
     in listOf("no", "false", "off", "n") -> false

compiler/cli/cli-js/src/org/jetbrains/kotlin/cli/js/K2JsIrCompiler.kt

@@ -432,7 +432,7 @@ class K2JsIrCompiler : CLICompiler<K2JSCompilerArguments>() {
 fun DceRuntimeDiagnostic.Companion.resolve(
     value: String?,
     messageCollector: MessageCollector
-): DceRuntimeDiagnostic? = when (value?.toLowerCase()) {
+): DceRuntimeDiagnostic? = when (value?.lowercase()) {
     DCE_RUNTIME_DIAGNOSTIC_LOG -> DceRuntimeDiagnostic.LOG
     DCE_RUNTIME_DIAGNOSTIC_EXCEPTION -> DceRuntimeDiagnostic.EXCEPTION
     null -> null

compiler/compiler-runner/src/org/jetbrains/kotlin/compilerRunner/CompilerOutputParser.kt

@@ -125,7 +125,7 @@ object CompilerOutputParser {
                 // We're directly inside the root tag: <MESSAGES>
                 return
             }
-            val qNameLowerCase = qName.toLowerCase(Locale.US)
+            val qNameLowerCase = qName.lowercase()
             var category: CompilerMessageSeverity? = CATEGORIES[qNameLowerCase]
             if (category == null) {
                 messageCollector.report(ERROR, "Unknown compiler message tag: $qName")

compiler/daemon/daemon-common/src/org/jetbrains/kotlin/daemon/common/ClientUtils.kt

@@ -107,7 +107,7 @@ private inline fun tryConnectToDaemon(port: Int, report: (DaemonReportCategory,
 private const val validFlagFileKeywordChars = "abcdefghijklmnopqrstuvwxyz0123456789-_"
 
 fun makeAutodeletingFlagFile(keyword: String = "compiler-client", baseDir: File? = null): File {
-    val prefix = "kotlin-${keyword.filter { validFlagFileKeywordChars.contains(it.toLowerCase()) }}-"
+    val prefix = "kotlin-${keyword.filter { validFlagFileKeywordChars.contains(it.lowercaseChar()) }}-"
     val flagFile = if (baseDir?.isDirectory == true)
         Files.createTempFile(baseDir.toPath(), prefix, "-is-running").toFile()
     else

compiler/daemon/daemon-common/src/org/jetbrains/kotlin/daemon/common/DaemonParams.kt

@@ -352,7 +352,7 @@ private val humanizedMemorySizeRegex = "(\\d+)([kmg]?)".toRegex()
 
 private fun String.memToBytes(): Long? =
         humanizedMemorySizeRegex
-            .matchEntire(this.trim().toLowerCase())
+            .matchEntire(this.trim().lowercase())
             ?.groups?.let { match ->
                 match[1]?.value?.let {
                     it.toLong() *

compiler/daemon/src/org/jetbrains/kotlin/daemon/LazyClasspathWatcher.kt

@@ -114,7 +114,7 @@ class LazyClasspathWatcher(classpath: Iterable<String>,
 }
 
 
-fun isClasspathFile(file: File): Boolean = file.isFile && listOf("class", "jar").contains(file.extension.toLowerCase())
+fun isClasspathFile(file: File): Boolean = file.isFile && listOf("class", "jar").contains(file.extension.lowercase())
 
 fun File.md5Digest(): ByteArray {
     val md = MessageDigest.getInstance(CLASSPATH_FILE_ID_DIGEST)

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/Generator.kt

@@ -160,10 +160,10 @@ class Generator(
         get() = "val $fieldName: $setType"
 
     private val Alias.fieldName: String
-        get() = removePrefix("Fir").decapitalize() + "s"
+        get() = removePrefix("Fir").replaceFirstChar(Char::lowercaseChar) + "s"
 
     private val Alias.allFieldName: String
-        get() = "all${fieldName.capitalize()}"
+        get() = "all${fieldName.replaceFirstChar(Char::uppercaseChar)}"
 
     private val Alias.setType: String
         get() = "Set<$this>"

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/ErrorListDiagnosticListRenderer.kt

@@ -124,7 +124,7 @@ object ErrorListDiagnosticListRenderer : DiagnosticListRenderer() {
         get() = classifier as KClass<*>
 
     private fun DiagnosticData.getFactoryFunction(): String =
-        severity.name.toLowerCase() + parameters.size
+        severity.name.lowercase() + parameters.size
 }
 
 private inline fun <T> SmartPrinter.printSeparatedWithComma(list: List<T>, printItem: (T) -> Unit) {

compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt

@@ -520,7 +520,7 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
 
     private fun FlowContent.modality(modality: Modality?) {
         if (modality == null) return
-        keyword(modality.name.toLowerCase())
+        keyword(modality.name.lowercase())
     }
 
     private fun FlowContent.visibility(visibility: Visibility) {

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/fir/modifier/Modifier.kt

@@ -41,7 +41,7 @@ class Modifier(
             parameterModifiers += ParameterModifier.CONST
             return
         }
-        val upperCasedModifier = modifier.toString().toUpperCase()
+        val upperCasedModifier = modifier.toString().uppercase()
         when {
             INLINE_MODIFIER.contains(tokenType) -> {
                 if (isInClass)

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/fir/modifier/TypeParameterModifier.kt

@@ -20,9 +20,9 @@ class TypeParameterModifier(
     fun addModifier(modifier: LighterASTNode) {
         val tokenType = modifier.tokenType
         when {
-            VARIANCE_MODIFIER.contains(tokenType) -> this.varianceModifiers += VarianceModifier.valueOf(modifier.toString().toUpperCase())
+            VARIANCE_MODIFIER.contains(tokenType) -> this.varianceModifiers += VarianceModifier.valueOf(modifier.toString().uppercase())
             REIFICATION_MODIFIER.contains(tokenType) -> this.reificationModifier =
-                ReificationModifier.valueOf(modifier.toString().toUpperCase())
+                ReificationModifier.valueOf(modifier.toString().uppercase())
         }
     }
 

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/fir/modifier/TypeProjectionModifier.kt

@@ -20,7 +20,7 @@ class TypeProjectionModifier(
     fun addModifier(modifier: LighterASTNode) {
         val tokenType = modifier.tokenType
         when {
-            VARIANCE_MODIFIER.contains(tokenType) -> this.varianceModifiers += VarianceModifier.valueOf(modifier.toString().toUpperCase())
+            VARIANCE_MODIFIER.contains(tokenType) -> this.varianceModifiers += VarianceModifier.valueOf(modifier.toString().uppercase())
         }
     }
 

compiler/fir/raw-fir/light-tree2fir/tests/org/jetbrains/kotlin/fir/lightTree/TotalKotlinTest.kt

@@ -82,7 +82,7 @@ class TotalKotlinTest : AbstractRawFirBuilderTestCase() {
             if (file.isDirectory) continue
             /* TODO: fix this, please !!! */
             if (file.path.contains("kotlin-native") ||
-                file.path.toLowerCase().contains("testdata") ||
+                file.path.lowercase().contains("testdata") ||
                 file.path.contains("resources")
             ) continue
             if (file.extension != "kt") continue

compiler/fir/raw-fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/RawFirBuilderTotalKotlinTestCase.kt

@@ -52,7 +52,7 @@ class RawFirBuilderTotalKotlinTestCase : AbstractRawFirBuilderTestCase() {
         println("BASE PATH: $testDataPath")
         for (file in root.walkTopDown()) {
             if (file.isDirectory) continue
-            val path = file.path.toLowerCase()
+            val path = file.path.lowercase()
             if ("testdata" in path ||
                 "kotlin-native" in path ||
                 "resources" in path ||
@@ -180,7 +180,7 @@ class RawFirBuilderTotalKotlinTestCase : AbstractRawFirBuilderTestCase() {
         val root = File(testDataPath)
         for (file in root.walkTopDown()) {
             if (file.isDirectory) continue
-            val path = file.path.toLowerCase()
+            val path = file.path.lowercase()
             if ("kotlin-native" in path ||
                 "testdata" in path ||
                 "resources" in path ||
@@ -211,7 +211,7 @@ class RawFirBuilderTotalKotlinTestCase : AbstractRawFirBuilderTestCase() {
         var counter = 0
         for (file in root.walkTopDown()) {
             if (file.isDirectory) continue
-            val path = file.path.toLowerCase()
+            val path = file.path.lowercase()
             if ("kotlin-native" in path ||
                 "testdata" in path ||
                 "resources" in path ||

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/context/AbstractBuilderConfigurator.kt

@@ -131,7 +131,7 @@ abstract class AbstractBuilderConfigurator<T : AbstractFirTreeBuilder>(val firTr
             thisRef: Nothing?,
             prop: KProperty<*>
         ): ReadOnlyProperty<Nothing?, IntermediateBuilder> {
-            val name = name ?: "Fir${prop.name.capitalize()}"
+            val name = name ?: "Fir${prop.name.replaceFirstChar(Char::uppercaseChar)}"
             builder = IntermediateBuilder(name).apply {
                 firTreeBuilder.intermediateBuilders += this
                 IntermediateBuilderConfigurationContext(this).block()

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/context/AbstractFieldConfigurator.kt

@@ -22,7 +22,7 @@ abstract class AbstractFieldConfigurator<T : AbstractFirTreeBuilder>(private val
 
         fun generateBooleanFields(vararg names: String) {
             names.forEach {
-                +booleanField("is${it.capitalize()}")
+                +booleanField("is${it.replaceFirstChar(Char::uppercaseChar)}")
             }
         }
 

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/ElementUtils.kt

@@ -27,7 +27,7 @@ fun field(name: String, typeWithArgs: Pair<Type, List<Importable>>, nullable: Bo
 }
 
 fun field(type: Type, nullable: Boolean = false, withReplace: Boolean = false): Field {
-    return SimpleField(type.type.decapitalize(), type.typeWithArguments, type.packageName, null, nullable, withReplace)
+    return SimpleField(type.type.replaceFirstChar(Char::lowercaseChar), type.typeWithArguments, type.packageName, null, nullable, withReplace)
 }
 
 fun booleanField(name: String, withReplace: Boolean = false): Field {
@@ -57,7 +57,7 @@ fun field(name: String, element: AbstractElement, nullable: Boolean = false, wit
 }
 
 fun field(element: Element, nullable: Boolean = false, withReplace: Boolean = false): Field {
-    return FirField(element.name.decapitalize(), element, nullable, withReplace)
+    return FirField(element.name.replaceFirstChar(Char::lowercaseChar), element, nullable, withReplace)
 }
 
 // ----------- Field list -----------
@@ -67,7 +67,7 @@ fun fieldList(name: String, type: Importable, withReplace: Boolean = false): Fie
 }
 
 fun fieldList(element: Element, withReplace: Boolean = false): Field {
-    return FieldList(element.name.decapitalize() + "s", element, withReplace)
+    return FieldList(element.name.replaceFirstChar(Char::lowercaseChar) + "s", element, withReplace)
 }
 
 // ----------- Field set -----------

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/printer/implementation.kt

@@ -228,7 +228,7 @@ fun SmartPrinter.printImplementation(implementation: Implementation) {
 
                             field.needsSeparateTransform -> {
                                 if (!(element.needTransformOtherChildren && field.needTransformInOtherChildren)) {
-                                    println("transform${field.name.capitalize()}(transformer, data)")
+                                    println("transform${field.name.replaceFirstChar(Char::uppercaseChar)}(transformer, data)")
                                 }
                             }
 
@@ -297,7 +297,7 @@ fun SmartPrinter.printImplementation(implementation: Implementation) {
                                 field.transform()
                             }
                             if (field.needTransformInOtherChildren) {
-                                println("transform${field.name.capitalize()}(transformer, data)")
+                                println("transform${field.name.replaceFirstChar(Char::uppercaseChar)}(transformer, data)")
                             }
                         }
                         println("return this")
@@ -335,7 +335,7 @@ fun SmartPrinter.printImplementation(implementation: Implementation) {
             }
 
             for (field in allFields.filter { it.withReplace }) {
-                val capitalizedFieldName = field.name.capitalize()
+                val capitalizedFieldName = field.name.replaceFirstChar(Char::uppercaseChar)
                 val newValue = "new$capitalizedFieldName"
                 generateReplace(field, forceNullable = field.useNullableForReplace) {
                     when {

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/printer/utils.kt

@@ -96,7 +96,7 @@ val Field.isVal: Boolean get() = this is FieldList || (this is FieldWithDefault
 
 
 fun Field.transformFunctionDeclaration(returnType: String): String {
-    return transformFunctionDeclaration(name.capitalize(), returnType)
+    return transformFunctionDeclaration(name.replaceFirstChar(Char::uppercaseChar), returnType)
 }
 
 fun transformFunctionDeclaration(transformName: String, returnType: String): String {
@@ -104,7 +104,7 @@ fun transformFunctionDeclaration(transformName: String, returnType: String): Str
 }
 
 fun Field.replaceFunctionDeclaration(overridenType: Importable? = null, forceNullable: Boolean = false): String {
-    val capName = name.capitalize()
+    val capName = name.replaceFirstChar(Char::uppercaseChar)
     val type = overridenType?.typeWithArguments ?: typeWithArguments
 
     val typeWithNullable = if (forceNullable && !type.endsWith("?")) "$type?" else type
@@ -136,7 +136,7 @@ fun Implementation.Kind?.braces(): String = when (this) {
     else -> throw IllegalStateException(this.toString())
 }
 
-val Element.safeDecapitalizedName: String get() = if (name == "Class") "klass" else name.decapitalize()
+val Element.safeDecapitalizedName: String get() = if (name == "Class") "klass" else name.replaceFirstChar(Char::lowercaseChar)
 
 val Importable.typeWithArguments: String
     get() = when (this) {

compiler/frontend.java/src/org/jetbrains/kotlin/load/kotlin/PackagePartClassUtils.kt

@@ -28,7 +28,7 @@ import java.util.*
 
 object PackagePartClassUtils {
     @JvmStatic fun getPathHashCode(file: VirtualFile): Int =
-            file.path.toLowerCase().hashCode()
+            file.path.lowercase().hashCode()
 
     private val PART_CLASS_NAME_SUFFIX = "Kt"
 
@@ -36,7 +36,7 @@ object PackagePartClassUtils {
             // NB use Locale.ENGLISH so that build is locale-independent.
             // See Javadoc on java.lang.String.toUpperCase() for more details.
             when {
-                Character.isJavaIdentifierStart(str[0]) -> str.substring(0, 1).toLowerCase(Locale.ENGLISH) + str.substring(1)
+                Character.isJavaIdentifierStart(str[0]) -> str.substring(0, 1).lowercase() + str.substring(1)
                 str[0] == '_' -> str.substring(1)
                 else -> str
             }

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/Renderers.kt

@@ -128,7 +128,7 @@ object Renderers {
         else
             declarationWithNameAndKind
 
-        withPlatform.capitalize()
+        withPlatform.replaceFirstChar(Char::uppercaseChar)
     }
 
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/diagnostics/KotlinSuppressCache.kt

@@ -51,7 +51,7 @@ abstract class KotlinSuppressCache {
     }
 
     fun isSuppressed(psiElement: PsiElement, suppressionKey: String, severity: Severity) =
-        isSuppressed(StringSuppressRequest(psiElement, severity, suppressionKey.toLowerCase()))
+        isSuppressed(StringSuppressRequest(psiElement, severity, suppressionKey.lowercase()))
 
     private fun isSuppressed(request: SuppressRequest): Boolean {
         // If diagnostics are reported in a synthetic file generated by KtPsiFactory (dummy.kt),
@@ -158,7 +158,7 @@ abstract class KotlinSuppressCache {
             if (arrayValue is ArrayValue) {
                 for (value in arrayValue.value) {
                     if (value is StringValue) {
-                        builder.add(value.value.toLowerCase())
+                        builder.add(value.value.lowercase())
                     }
                 }
             }
@@ -167,7 +167,7 @@ abstract class KotlinSuppressCache {
 
     companion object {
         private fun getDiagnosticSuppressKey(diagnostic: Diagnostic): String =
-            diagnostic.factory.name.toLowerCase()
+            diagnostic.factory.name.lowercase()
 
         private fun isSuppressedByStrings(key: String, strings: Set<String>, severity: Severity): Boolean =
             severity == Severity.WARNING && "warnings" in strings || key in strings

compiler/incremental-compilation-impl/src/org/jetbrains/kotlin/incremental/IncrementalJvmCompilerRunner.kt

@@ -65,7 +65,7 @@ fun makeIncrementally(
     val allExtensions = kotlinExtensions + "java"
     val rootsWalk = sourceRoots.asSequence().flatMap { it.walk() }
     val files = rootsWalk.filter(File::isFile)
-    val sourceFiles = files.filter { it.extension.toLowerCase() in allExtensions }.toList()
+    val sourceFiles = files.filter { it.extension.lowercase() in allExtensions }.toList()
     val buildHistoryFile = File(cachesDir, "build-history.bin")
     args.javaSourceRoots = sourceRoots.map { it.absolutePath }.toTypedArray()
     val buildReporter = BuildReporter(icReporter = reporter, buildMetricsReporter = DoNothingBuildMetricsReporter)
@@ -275,7 +275,7 @@ class IncrementalJvmCompilerRunner(
     private fun processLookupSymbolsForAndroidLayouts(changedFiles: ChangedFiles.Known): Collection<LookupSymbol> {
         val result = mutableListOf<LookupSymbol>()
         for (file in changedFiles.modified + changedFiles.removed) {
-            if (file.extension.toLowerCase() != "xml") continue
+            if (file.extension.lowercase() != "xml") continue
             val layoutName = file.name.substringBeforeLast('.')
             result.add(LookupSymbol(ANDROID_LAYOUT_CONTENT_LOOKUP_NAME, layoutName))
         }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/dumpKotlinLike.kt

@@ -283,7 +283,7 @@ private class KotlinLikeDumper(val p: Printer, val options: KotlinLikeDumpOption
             else ->
                 Modality.FINAL
         }
-        p(modality, defaultModality) { name.toLowerCase() }
+        p(modality, defaultModality) { name.lowercase() }
         p(isExternal, "external")
         p(isFakeOverride, customModifier("fake"))
         p(isOverride, "override")
@@ -302,7 +302,7 @@ private class KotlinLikeDumper(val p: Printer, val options: KotlinLikeDumpOption
         p(isData, "data")
         p(isCompanion, "companion")
         p(isFunInterface, "fun")
-        p(classKind) { name.toLowerCase().replace('_', ' ') + if (this == ClassKind.ENUM_ENTRY) " class" else "" }
+        p(classKind) { name.lowercase().replace('_', ' ') + if (this == ClassKind.ENUM_ENTRY) " class" else "" }
         p(isInfix, "infix")
         p(isOperator, "operator")
     }

compiler/psi/src/org/jetbrains/kotlin/parsing/ParseUtils.kt

@@ -79,7 +79,7 @@ private fun parseLong(text: String): Long? {
 }
 
 private fun parseFloatingLiteral(text: String): Number? {
-    if (text.toLowerCase().endsWith('f')) {
+    if (text.lowercase().endsWith('f')) {
         return parseFloat(text)
     }
     return parseDouble(text)

compiler/psi/src/org/jetbrains/kotlin/psi/stubs/impl/KotlinStubBaseImpl.kt

@@ -72,7 +72,7 @@ open class KotlinStubBaseImpl<T : KtElementImplStub<*>>(parent: StubElement<*>?,
     private fun getPropertyName(method: Method): String {
         val methodName = method.name!!
         if (methodName.startsWith("get")) {
-            return methodName.substring(3).decapitalize()
+            return methodName.substring(3).replaceFirstChar(Char::lowercaseChar)
         }
         return methodName
     }

compiler/testData/diagnostics/testsWithJsStdLib/unsafeCastFunctionOnDynamicType.kt

@@ -5,7 +5,7 @@ fun String.toDouble(): Double = (+(this.asDynamic())).unsafeCast<Double>().also
         TODO()
 }
 
-fun String.isNaN(): Boolean = when (this.toLowerCase()) {
+fun String.isNaN(): Boolean = when (this.lowercase()) {
     "nan", "+nan", "-nan" -> true
     else -> false
 }

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/services/impl/ModuleStructureExtractorImpl.kt

@@ -316,7 +316,7 @@ class ModuleStructureExtractorImpl(
         }
 
         private fun parseModulePlatformByName(moduleName: String): TargetPlatform? {
-            val nameSuffix = moduleName.substringAfterLast("-", "").toUpperCase()
+            val nameSuffix = moduleName.substringAfterLast("-", "").uppercase()
             return when {
                 nameSuffix == "COMMON" -> CommonPlatforms.defaultCommonPlatform
                 nameSuffix == "JVM" -> JvmPlatforms.unspecifiedJvmPlatform // TODO(dsavvinov): determine JvmTarget precisely

compiler/tests-common/tests/org/jetbrains/kotlin/checkers/BaseDiagnosticsTest.kt

@@ -536,7 +536,7 @@ abstract class BaseDiagnosticsTest : KotlinMultiFileTestWithJava<TestModule, Tes
     private fun parseJvmTarget(directiveMap: Directives) = directiveMap[JVM_TARGET]?.let { JvmTarget.fromString(it) }
 
     protected fun parseModulePlatformByName(moduleName: String): TargetPlatform? {
-        val nameSuffix = moduleName.substringAfterLast("-", "").toUpperCase()
+        val nameSuffix = moduleName.substringAfterLast("-", "").uppercase()
         return when {
             nameSuffix == "COMMON" -> CommonPlatforms.defaultCommonPlatform
             nameSuffix == "JVM" -> JvmPlatforms.unspecifiedJvmPlatform // TODO(dsavvinov): determine JvmTarget precisely

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/36.kt

@@ -19,14 +19,14 @@
 fun case_1(x: Class?, y: Any) {
     x?.prop_12 = if (y is String) "" else throw Exception()
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }
 
 // TESTCASE NUMBER: 2
 fun case_2(x: Class?, y: Any) {
     x?.prop_9 = y is String || return
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any")!>y<!>.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>toUpperCase<!>()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any")!>y<!>.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>uppercase<!>()
 }
 
 /*
@@ -37,7 +37,7 @@ fun case_2(x: Class?, y: Any) {
 fun case_3(x: Class?, y: Any) {
     x?.prop_12 = y as String
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }
 
 /*
@@ -48,7 +48,7 @@ fun case_3(x: Class?, y: Any) {
 fun case_4(x: Class?, y: Any) {
     x?.prop_12 = y as? String ?: return
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any & kotlin.String"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }
 
 /*
@@ -59,14 +59,14 @@ fun case_4(x: Class?, y: Any) {
 fun case_5(x: Class?, y: String?) {
     x?.prop_12 = y ?: return
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }
 
 // TESTCASE NUMBER: 6
 fun case_6(x: Class?, y: String?) {
     x?.prop_9 = y !is String && throw Exception()
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>y<!><!UNSAFE_CALL!>.<!>toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>y<!><!UNSAFE_CALL!>.<!>uppercase()
 }
 
 /*
@@ -77,7 +77,7 @@ fun case_6(x: Class?, y: String?) {
 fun case_7(x: Class?, y: String?) {
     x?.prop_12 = y!!
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }
 
 /*
@@ -88,5 +88,5 @@ fun case_7(x: Class?, y: String?) {
 fun case_8(x: Class?, y: String?) {
     x?.prop_12 = if (y === null) throw Exception() else ""
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?")!>y<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.toUpperCase()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String & kotlin.String?"), DEBUG_INFO_SMARTCAST!>y<!>.uppercase()
 }

compiler/tests-spec/tests/org/jetbrains/kotlin/spec/utils/TestsJsonMapGenerator.kt

@@ -41,7 +41,7 @@ object TestsJsonMapGenerator {
         SECONDARY;
 
         override fun toString(): String {
-            return name.toLowerCase()
+            return name.lowercase()
         }
     }
 

compiler/tests-spec/tests/org/jetbrains/kotlin/spec/utils/TestsStatisticCollector.kt

@@ -43,7 +43,7 @@ object TestsStatisticCollector {
         val statistic = mutableMapOf<TestArea, SpecTestsStatElement>()
 
         for (specTestArea in TestArea.values()) {
-            val specTestsPath = "$SPEC_TESTDATA_PATH/${specTestArea.name.toLowerCase().replace("_", "/")}/${testLinkedType.testDataPath}"
+            val specTestsPath = "$SPEC_TESTDATA_PATH/${specTestArea.name.lowercase().replace("_", "/")}/${testLinkedType.testDataPath}"
 
             statistic[specTestArea] =
                 SpecTestsStatElement(SpecTestsStatElementType.AREA)

compiler/tests-spec/tests/org/jetbrains/kotlin/spec/utils/parsers/CommonParser.kt

@@ -23,7 +23,7 @@ import java.util.regex.Pattern
 object CommonParser {
     fun String.withUnderscores() = replace(" ", "_")
         .replace(File.separator, "_")
-        .toUpperCase()
+        .uppercase()
 
     fun String.splitByComma() = split(Regex(""",\s*"""))
     fun String.splitByPathSeparator() = split(File.separator)

compiler/tests-spec/tests/org/jetbrains/kotlin/spec/utils/parsers/TestInfoParser.kt

@@ -85,8 +85,8 @@ private fun parseImplementationTestInfo(testFilePath: String, linkedTestType: Sp
         testArea = TestArea.valueOf(testInfoByContentMatcher.group("testArea").withUnderscores()),
         testType = TestType.valueOf(testInfoByContentMatcher.group("testType")),
         testNumber = testInfoElements[CommonSpecTestFileInfoElementType.NUMBER]?.content?.toInt() ?: 0,
-        testDescription = fileNameWithoutExtension.toUpperCase()[0] + fileNameWithoutExtension.substring(1)
+        testDescription = fileNameWithoutExtension.uppercase()[0] + fileNameWithoutExtension.substring(1)
-            .replace(Regex("""([A-Z])"""), " $1").toLowerCase(),
+            .replace(Regex("""([A-Z])"""), " $1").lowercase(),
         testInfoElements = testInfoElements,
         testCasesSet = SpecTestCasesSet(mutableMapOf(), mutableMapOf(), mutableMapOf()), //todo
         unexpectedBehavior = testInfoElements.contains(CommonInfoElementType.UNEXPECTED_BEHAVIOUR),

compiler/tests-spec/tests/org/jetbrains/kotlin/spec/utils/spec/HtmlSpecSentencesMapBuilder.kt

@@ -24,7 +24,7 @@ object HtmlSpecSentencesMapBuilder {
         spec.select("$SECTION_SELECTORS, $PARAGRAPH_SELECTORS, .sentence").forEach { element ->
             when {
                 element.`is`(SECTION_SELECTORS) -> {
-                    val sectionTag = SectionTag.valueOf(element.tagName().toLowerCase())
+                    val sectionTag = SectionTag.valueOf(element.tagName().lowercase())
                     while (!currentSectionsPath.empty() && currentSectionsPath.peek().first.level >= sectionTag.level) {
                         currentSectionsPath.pop()
                     }

compiler/tests/org/jetbrains/kotlin/generators/tests/GenerateSteppedRangesCodegenTestData.kt

@@ -242,7 +242,7 @@ object GenerateSteppedRangesCodegenTestData {
 
     private fun PrintWriter.printTestForFunctionAndType(builder: TestBuilder, function: Function, type: Type, asLiteral: Boolean) {
         val shouldFail = (builder.expectedValuesOrFailIfNull == null)
-        val listVarName = type.type.toLowerCase() + "List"
+        val listVarName = type.type.lowercase() + "List"
         if (shouldFail) {
             println("    assertFailsWith<IllegalArgumentException> {")
         } else {
@@ -253,7 +253,7 @@ object GenerateSteppedRangesCodegenTestData {
         if (asLiteral) {
             println("$indent    for (i in ${builder.buildFullLiteral(type, function)}) {")
         } else {
-            val progressionVarName = type.type.toLowerCase() + "Progression"
+            val progressionVarName = type.type.lowercase() + "Progression"
             println("$indent    val $progressionVarName = ${builder.buildRangeOnlyExpression(type, function)}")
             println("$indent    for (i in ${builder.buildOnTopOfRangeOnlyVariable(progressionVarName, type)}) {")
         }

compiler/tests/org/jetbrains/kotlin/jvm/compiler/MultiModuleJavaAnalysisCustomTest.kt

@@ -86,7 +86,7 @@ class MultiModuleJavaAnalysisCustomTest : KtUsefulTestCase() {
         val platformParameters = JvmPlatformParameters(
             packagePartProviderFactory = { PackagePartProvider.Empty },
             moduleByJavaClass = { javaClass ->
-                val moduleName = javaClass.name.asString().toLowerCase().first().toString()
+                val moduleName = javaClass.name.asString().lowercase().first().toString()
                 modules.first { it._name == moduleName }
             },
             useBuiltinsProviderForModule = { false }
@@ -165,8 +165,8 @@ class MultiModuleJavaAnalysisCustomTest : KtUsefulTestCase() {
             module ->
             val moduleDescriptor = resolverForProject.descriptorForModule(module)
 
-            checkClassInPackage(moduleDescriptor, "test", "Kotlin${module._name.toUpperCase()}")
+            checkClassInPackage(moduleDescriptor, "test", "Kotlin${module._name.uppercase()}")
-            checkClassInPackage(moduleDescriptor, "custom", "${module._name.toUpperCase()}Class")
+            checkClassInPackage(moduleDescriptor, "custom", "${module._name.uppercase()}Class")
         }
     }
 
@@ -229,7 +229,7 @@ class MultiModuleJavaAnalysisCustomTest : KtUsefulTestCase() {
         assert(!ErrorUtils.isError(referencedDescriptor)) { "Error descriptor: $referencedDescriptor" }
 
         val descriptorName = referencedDescriptor.name.asString()
-        val expectedModuleName = "<${descriptorName.toLowerCase().first()}>"
+        val expectedModuleName = "<${descriptorName.lowercase().first()}>"
         val moduleName = referencedDescriptor.module.name.asString()
         Assert.assertEquals(
                 "Java class $descriptorName in $context should be in module $expectedModuleName, but instead was in $moduleName",

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -206,7 +206,7 @@ enum class LanguageFeature(
 
     val presentableName: String
     // E.g. "DestructuringLambdaParameters" -> ["Destructuring", "Lambda", "Parameters"] -> "destructuring lambda parameters"
-        get() = name.split("(?<!^)(?=[A-Z])".toRegex()).joinToString(separator = " ", transform = String::toLowerCase)
+        get() = name.split("(?<!^)(?=[A-Z])".toRegex()).joinToString(separator = " ", transform = String::lowercase)
 
     val presentableText get() = if (hintUrl == null) presentableName else "$presentableName (See: $hintUrl)"
 

compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt

@@ -827,7 +827,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
             val name = arrayOfCall.typeRef.coneType.classId!!.shortClassName.asString()
             val typeArguments = arrayOfCall.typeRef.coneType.typeArguments
             val typeParameters = if (typeArguments.isEmpty()) "" else " <T>"
-            data.append("fun$typeParameters ${name.decapitalize()}Of")
+            data.append("fun$typeParameters ${name.replaceFirstChar(Char::lowercaseChar)}Of")
             typeArguments.firstOrNull()?.let {
                 data.append("<").append(it.tryToRenderConeAsFunctionType()).append(">")
             }

core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt

@@ -39,10 +39,10 @@ object NameUtils {
 
     @JvmStatic
     private fun capitalizeAsJavaClassName(str: String): String =
-            // NB use Locale.ENGLISH so that build is locale-independent.
+            // NB `uppercase` uses Locale.ROOT and is locale-independent.
             // See Javadoc on java.lang.String.toUpperCase() for more details.
             if (Character.isJavaIdentifierStart(str[0]))
-                str.substring(0, 1).toUpperCase(Locale.ENGLISH) + str.substring(1)
+                str.substring(0, 1).uppercase() + str.substring(1)
             else
                 "_$str"
 

core/descriptors/src/org/jetbrains/kotlin/builtins/functions/FunctionInvokeDescriptor.kt

@@ -138,7 +138,7 @@ class FunctionInvokeDescriptor private constructor(
                 "E" -> "receiver"
                 else -> {
                     // Type parameter "P1" -> value parameter "p1", "P2" -> "p2", etc.
-                    typeParameterName.toLowerCase()
+                    typeParameterName.lowercase()
                 }
             }
 

core/descriptors/src/org/jetbrains/kotlin/renderer/DescriptorRendererOptionsImpl.kt

@@ -48,7 +48,7 @@ internal class DescriptorRendererOptionsImpl : DescriptorRendererOptions {
             assert(!field.name.startsWith("is")) { "Fields named is* are not supported here yet" }
             val value = property.getValue(
                     this,
-                    PropertyReference1Impl(DescriptorRendererOptionsImpl::class, field.name, "get" + field.name.capitalize())
+                    PropertyReference1Impl(DescriptorRendererOptionsImpl::class, field.name, "get" + field.name.replaceFirstChar(Char::uppercaseChar))
             )
             field.set(copy, copy.property(value))
         }

core/util.runtime/src/org/jetbrains/kotlin/utils/capitalizeDecapitalize.kt

@@ -28,7 +28,7 @@ fun String.decapitalizeSmartForCompiler(asciiOnly: Boolean = false): String {
     if (isEmpty() || !isUpperCaseCharAt(0, asciiOnly)) return this
 
     if (length == 1 || !isUpperCaseCharAt(1, asciiOnly)) {
-        return if (asciiOnly) decapitalizeAsciiOnly() else decapitalize()
+        return if (asciiOnly) decapitalizeAsciiOnly() else replaceFirstChar(Char::lowercaseChar)
     }
 
     val secondWordStart = (indices.firstOrNull { !isUpperCaseCharAt(it, asciiOnly) } ?: return toLowerCase(this, asciiOnly)) - 1
@@ -98,18 +98,18 @@ private fun String.isLowerCaseCharAt(index: Int, asciiOnly: Boolean): Boolean {
 }
 
 private fun toLowerCase(string: String, asciiOnly: Boolean): String {
-    return if (asciiOnly) string.toLowerCaseAsciiOnly() else string.toLowerCase()
+    return if (asciiOnly) string.toLowerCaseAsciiOnly() else string.lowercase()
 }
 
 private fun toUpperCase(string: String, asciiOnly: Boolean): String {
-    return if (asciiOnly) string.toUpperCaseAsciiOnly() else string.toUpperCase()
+    return if (asciiOnly) string.toUpperCaseAsciiOnly() else string.uppercase()
 }
 
 fun String.capitalizeAsciiOnly(): String {
     if (isEmpty()) return this
     val c = this[0]
     return if (c in 'a'..'z')
-        c.toUpperCase() + substring(1)
+        c.uppercaseChar() + substring(1)
     else
         this
 }
@@ -118,7 +118,7 @@ fun String.decapitalizeAsciiOnly(): String {
     if (isEmpty()) return this
     val c = this[0]
     return if (c in 'A'..'Z')
-        c.toLowerCase() + substring(1)
+        c.lowercaseChar() + substring(1)
     else
         this
 }
@@ -126,7 +126,7 @@ fun String.decapitalizeAsciiOnly(): String {
 fun String.toLowerCaseAsciiOnly(): String {
     val builder = StringBuilder(length)
     for (c in this) {
-        builder.append(if (c in 'A'..'Z') c.toLowerCase() else c)
+        builder.append(if (c in 'A'..'Z') c.lowercaseChar() else c)
     }
     return builder.toString()
 }
@@ -134,7 +134,7 @@ fun String.toLowerCaseAsciiOnly(): String {
 fun String.toUpperCaseAsciiOnly(): String {
     val builder = StringBuilder(length)
     for (c in this) {
-        builder.append(if (c in 'a'..'z') c.toUpperCase() else c)
+        builder.append(if (c in 'a'..'z') c.uppercaseChar() else c)
     }
     return builder.toString()
 }

generators/evaluate/GenerateOperationsMap.kt

@@ -188,7 +188,7 @@ private fun CallableDescriptor.getParametersTypes(): List<KotlinType> =
     listOf((containingDeclaration as ClassDescriptor).defaultType) +
             valueParameters.map { it.type.makeNotNullable() }
 
-private fun KotlinType.asString(): String = typeName.toUpperCase()
+private fun KotlinType.asString(): String = typeName.uppercase()
 
 private val KotlinType.typeName: String
     get(): String = constructor.declarationDescriptor!!.name.asString()

generators/test-generator/tests/org/jetbrains/kotlin/generators/model/SimpleTestMethodModel.kt

@@ -46,10 +46,10 @@ open class SimpleTestMethodModel(
                 extractedName
             } else {
                 val relativePath = FileUtil.getRelativePath(rootDir, file.parentFile)
-                relativePath + "-" + extractedName.capitalize()
+                relativePath + "-" + extractedName.replaceFirstChar(Char::uppercaseChar)
             }
             val ignored = skipIgnored && InTextDirectivesUtils.isIgnoredTarget(targetBackend, file)
-            return (if (ignored) "ignore" else "test") + escapeForJavaIdentifier(unescapedName).capitalize()
+            return (if (ignored) "ignore" else "test") + escapeForJavaIdentifier(unescapedName).replaceFirstChar(Char::uppercaseChar)
         }
 
     init {

generators/test-generator/tests/org/jetbrains/kotlin/generators/util/TestGeneratorUtil.kt

@@ -31,7 +31,7 @@ object TestGeneratorUtil {
 
     @JvmStatic
     fun fileNameToJavaIdentifier(file: File): String {
-        return escapeForJavaIdentifier(file.name).capitalize()
+        return escapeForJavaIdentifier(file.name).replaceFirstChar(Char::uppercaseChar)
     }
 }
 

generators/tests/org/jetbrains/kotlin/generators/protobuf/GenerateProtoBufCompare.kt

@@ -89,7 +89,7 @@ class GenerateProtoBufCompare {
             }
             return (extensionPrefix + fieldDescriptor.name.javaName + (if (fieldDescriptor.isRepeated) "List" else ""))
                 .replace("[A-Z]".toRegex()) { "_" + it.value }
-                .toUpperCase()
+                .uppercase()
         }
     }
 
@@ -236,7 +236,7 @@ class GenerateProtoBufCompare {
 
     fun generateHashCodeForField(field: Descriptors.FieldDescriptor, p: Printer, isExtensionField: Boolean) {
         val fieldName = field.name.javaName
-        val capFieldName = fieldName.capitalize()
+        val capFieldName = fieldName.replaceFirstChar(Char::uppercaseChar)
         val outerClassName = field.file.options.javaOuterClassname.removePrefix("Debug")
         val fullFieldName = "$outerClassName.$fieldName"
 
@@ -310,7 +310,7 @@ class GenerateProtoBufCompare {
 
         val typeName = field.containingType.typeName
         val fieldName = field.name.javaName
-        val capFieldName = fieldName.capitalize()
+        val capFieldName = fieldName.replaceFirstChar(Char::uppercaseChar)
         val methodName = field.helperMethodName()
 
         p.println()
@@ -357,7 +357,7 @@ class GenerateProtoBufCompare {
 
     open inner class FieldGeneratorImpl(field: Descriptors.FieldDescriptor, p: Printer) : FieldGenerator(field, p) {
         val fieldName = field.name.javaName
-        val capFieldName = fieldName.capitalize()
+        val capFieldName = fieldName.replaceFirstChar(Char::uppercaseChar)
 
         override fun printRepeatedField() {
             repeatedFields.add(field)
@@ -496,12 +496,12 @@ class GenerateProtoBufCompare {
         val packageHeader = this.file.`package`
         val descriptor = this.containingType
         val className = descriptor.fullName.removePrefix(packageHeader).replace(".", "")
-        val capFieldName = this.name.javaName.capitalize()
+        val capFieldName = this.name.javaName.replaceFirstChar(Char::uppercaseChar)
         return "$CHECK_EQUALS_NAME$className$capFieldName"
     }
 
     private val String.javaName: String
-        get() = this.split("_").joinToString("") { it.capitalize() }.decapitalize()
+        get() = this.split("_").joinToString("") { it.replaceFirstChar(Char::uppercaseChar) }.replaceFirstChar(Char::lowercaseChar)
 }
 
 private fun Printer.printlnMultiline(string: String) {

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/KDocCompletionContributor.kt

@@ -158,7 +158,7 @@ object KDocTagCompletionProvider : CompletionProvider<CompletionParameters>() {
         val resultWithPrefix = result.withPrefixMatcher(prefix)
         KDocKnownTag.values().forEach {
             if (kdocOwner == null || it.isApplicable(kdocOwner)) {
-                resultWithPrefix.addElement(LookupElementBuilder.create("@" + it.name.toLowerCase(Locale.US)))
+                resultWithPrefix.addElement(LookupElementBuilder.create("@" + it.name.lowercase()))
             }
         }
     }

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/VariableOrParameterNameWithTypeCompletion.kt

@@ -68,7 +68,7 @@ class VariableOrParameterNameWithTypeCompletion(
             prefixWords.indices.map { index -> if (index == 0) prefix else prefixWords.drop(index).joinToString("") }
 
         userPrefixes = nameSuggestionPrefixes.indices.map { prefixWords.take(it).joinToString("") }
-        classNamePrefixMatchers = nameSuggestionPrefixes.map { CamelHumpMatcher(it.capitalize(Locale.US), false) }
+        classNamePrefixMatchers = nameSuggestionPrefixes.map { CamelHumpMatcher(it.replaceFirstChar(Char::uppercaseChar), false) }
     }
 
     private val suggestionsByTypesAdded = HashSet<Type>()

idea/idea-completion/testData/basic/common/inStringLiterals/StringTemplateDot.kt

@@ -4,4 +4,4 @@ fun foo(param: String) {
 
 // EXIST: { itemText: "length", attributes: "" }
 // EXIST: { itemText: "hashCode", attributes: "" }
-// EXIST: { itemText: "capitalize", attributes: "bold" }
+// EXIST: { itemText: "lowercase", attributes: "bold" }

idea/idea-core/src/org/jetbrains/kotlin/idea/statistics/ProjectConfigurationCollector.kt

@@ -62,8 +62,8 @@ class ProjectConfigurationCollector : ProjectUsagesCollector() {
         val buildSystem = it.getBuildSystemType()
         return when {
             buildSystem == BuildSystemType.JPS -> "JPS"
-            buildSystem.toString().toLowerCase().contains("maven") -> "Maven"
+            buildSystem.toString().lowercase().contains("maven") -> "Maven"
-            buildSystem.toString().toLowerCase().contains("gradle") -> "Gradle"
+            buildSystem.toString().lowercase().contains("gradle") -> "Gradle"
             else -> "unknown"
         }
     }

idea/idea-frontend-fir/idea-fir-low-level-api/tests/org/jetbrains/kotlin/idea/fir/low/level/api/AbstractFirLazyDeclarationResolveTest.kt

@@ -31,7 +31,7 @@ abstract class AbstractFirLazyDeclarationResolveTest : KotlinLightCodeInsightFix
         val ktFile = myFixture.configureByText(testDataFile.name, FileUtil.loadFile(testDataFile)) as KtFile
         val lazyDeclarations = ktFile.collectDescendantsOfType<KtDeclaration> { ktDeclaration -> !KtPsiUtil.isLocal(ktDeclaration) }
 
-        val declarationToResolve = lazyDeclarations.firstOrNull { it.name?.toLowerCase() == "resolveme" }
+        val declarationToResolve = lazyDeclarations.firstOrNull { it.name?.lowercase() == "resolveme" }
             ?: error("declaration with name `resolveMe` was not found")
         resolveWithClearCaches(ktFile) { firModuleResolveState ->
             val rendered = declarationToResolve.withFirDeclaration(

idea/idea-frontend-fir/idea-frontend-fir-generator/src/org/jetbrains/kotlin/idea/frontend/api/fir/generator/HLDiagnosticConverter.kt

@@ -100,7 +100,7 @@ private object FirToKtConversionCreator {
         if (kClass.isSubclassOf(Collection::class)) {
             val elementType = type.arguments.single().type ?: return HLIdParameterConversion
             return HLMapParameterConversion(
-                parameterName = elementType.kClass.simpleName!!.decapitalize(),
+                parameterName = elementType.kClass.simpleName!!.replaceFirstChar(Char::lowercaseChar),
                 mappingConversion = createConversion(elementType)
             )
         }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/references/KtFirCollectionLiteralReference.kt

@@ -50,7 +50,7 @@ class KtFirCollectionLiteralReference(
         }.associateWith { it.correspondingArrayOfCallFqName() }
 
         private fun ClassId.correspondingArrayOfCallFqName(): Name =
-            Name.identifier("${shortClassName.identifier.decapitalize()}Of")
+            Name.identifier("${shortClassName.identifier.replaceFirstChar(Char::lowercaseChar)}Of")
 
     }
 }

idea/idea-gradle/src/org/jetbrains/kotlin/idea/configuration/KotlinBuildScriptManipulator.kt

@@ -376,7 +376,7 @@ class KotlinBuildScriptManipulator(
         findPlugin(pluginName) ?: addExpressionIfMissing("plugin(\"$pluginName\")") as? KtCallExpression
 
     private fun KtFile.changeCoroutineConfiguration(coroutineOption: String): PsiElement? {
-        val snippet = "experimental.coroutines = Coroutines.${coroutineOption.toUpperCase()}"
+        val snippet = "experimental.coroutines = Coroutines.${coroutineOption.uppercase()}"
         val kotlinBlock = getKotlinBlock() ?: return null
         addImportIfMissing("org.jetbrains.kotlin.gradle.dsl.Coroutines")
         val statement = kotlinBlock.statements.find { it.text.startsWith("experimental.coroutines") }

idea/idea-gradle/src/org/jetbrains/kotlin/idea/statistics/KotlinGradleFUSLogger.kt

@@ -50,7 +50,7 @@ class KotlinGradleFUSLogger : StartupActivity, DumbAware, Runnable {
                     // 2. the projectId should be stable and independent on IDE version
                     val presentableUrl = FileUtil.toSystemIndependentName(path)
                     val name =
-                        PathUtilRt.getFileName(presentableUrl).toLowerCase(Locale.US).removeSuffix(ProjectFileType.DOT_DEFAULT_EXTENSION)
+                        PathUtilRt.getFileName(presentableUrl).lowercase().removeSuffix(ProjectFileType.DOT_DEFAULT_EXTENSION)
                     val locationHash = Integer.toHexString((presentableUrl).hashCode())
                     val projectHash =
                         "${name.trimMiddle(name.length.coerceAtMost(254 - locationHash.length), useEllipsisSymbol = false)}.$locationHash"
@@ -85,7 +85,7 @@ class KotlinGradleFUSLogger : StartupActivity, DumbAware, Runnable {
             val data = HashMap<String, String>()
             fun putIfNotNull(key: String, value: String?) {
                 if (value != null) {
-                    data[key.toLowerCase()] = value
+                    data[key.lowercase()] = value
                 }
             }
 

idea/idea-gradle/tests/org/jetbrains/kotlin/idea/codeInsight/gradle/GradleTestRunConfigurationAndHighlightingTest.kt

@@ -127,7 +127,7 @@ class GradleTestRunConfigurationAndHighlightingTest : GradleImportingTestCase()
                 RunConfigurationsTags.SETTINGS -> settings.toString()
             }
 
-            result += tag.name.toLowerCase() to renderedTagValue
+            result += tag.name.lowercase() to renderedTagValue
         }
 
         return result.joinToString { (tagName, tagValue) -> tagName + "=\"" + tagValue.replace("\"", "\\\"") + "\"" }

idea/idea-jvm/src/org/jetbrains/kotlin/idea/compiler/KotlinCompilerManager.kt

@@ -74,7 +74,7 @@ class KotlinCompilerStartupActivity : StartupActivity {
             override fun fileGenerated(outputRoot: String, relativePath: String) {
                 if (isUnitTestMode()) return
 
-                val ext = FileUtilRt.getExtension(relativePath).toLowerCase()
+                val ext = FileUtilRt.getExtension(relativePath).lowercase()
                 if (FILE_EXTS_WHICH_NEEDS_REFRESH.contains(ext)) {
                     val outFile = "$outputRoot/$relativePath"
                     val virtualFile = LocalFileSystem.getInstance().findFileByPath(outFile)

idea/idea-new-project-wizard/src/org/jetbrains/kotlin/tools/projectWizard/wizard/NewProjectWizardModuleBuilder.kt

@@ -218,7 +218,7 @@ class ModuleNewWizardFirstStep(wizard: IdeWizard) : WizardStep(wizard, Generatio
     private fun suggestGroupId(): String {
         val username = SystemProperties.getUserName() ?: return DEFAULT_GROUP_ID
         if (!username.matches("[\\w\\s]+".toRegex())) return DEFAULT_GROUP_ID
-        val usernameAsGroupId = username.trim().toLowerCase(Locale.US).split("\\s+".toRegex()).joinToString(separator = ".")
+        val usernameAsGroupId = username.trim().lowercase().split("\\s+".toRegex()).joinToString(separator = ".")
         return "me.$usernameAsGroupId"
     }
 

idea/idea-new-project-wizard/src/org/jetbrains/kotlin/tools/projectWizard/wizard/ui/secondStep/modulesEditor/ModulesEditorToolbarDecorator.kt

@@ -73,7 +73,7 @@ class ModulesEditorToolbarDecorator(
                     null -> ""
                 }
 
-                text = KotlinNewProjectWizardUIBundle.message("editor.modules.add", moduleKindTextToAdd.capitalize(Locale.US))
+                text = KotlinNewProjectWizardUIBundle.message("editor.modules.add", moduleKindTextToAdd.replaceFirstChar(Char::uppercaseChar))
             }
             event.presentation.isEnabled
         }
@@ -104,7 +104,7 @@ class ModulesEditorToolbarDecorator(
                 isEnabled = tree.selectedSettingItem is Module
                 text = KotlinNewProjectWizardUIBundle.message(
                     "editor.modules.remove.tooltip",
-                    selectedModuleKindText?.let { " ${it.capitalize(Locale.US)}" }.orEmpty()
+                    selectedModuleKindText?.let { " ${it.replaceFirstChar { if (it.isLowerCase()) it.titlecase(Locale.US) else it.toString() }}" }.orEmpty()
                 )
             }
             event.presentation.isEnabled

idea/jvm-debugger/eval4j/test/org/jetbrains/eval4j/test/util.kt

@@ -16,4 +16,5 @@
 
 package org.jetbrains.eval4j.test
 
-fun getTestName(methodName: String) = if (methodName.startsWith("test")) methodName else "test${methodName.capitalize()}"
+fun getTestName(methodName: String) =
+    if (methodName.startsWith("test")) methodName else "test${methodName.replaceFirstChar(Char::uppercaseChar)}"

idea/jvm-debugger/jvm-debugger-core/src/org/jetbrains/kotlin/idea/debugger/debuggerUtil.kt

@@ -37,7 +37,7 @@ fun Location.isInKotlinSources(): Boolean {
 }
 
 fun ReferenceType.isInKotlinSources(): Boolean {
-    val fileExtension = safeSourceName()?.substringAfterLast('.')?.toLowerCase() ?: ""
+    val fileExtension = safeSourceName()?.substringAfterLast('.')?.lowercase() ?: ""
     return fileExtension in KotlinFileTypeFactoryUtils.KOTLIN_EXTENSIONS || containsKotlinStrata()
 }
 

idea/jvm-debugger/jvm-debugger-evaluation/src/org/jetbrains/kotlin/idea/debugger/evaluate/KotlinEvaluatorBuilder.kt

@@ -513,7 +513,7 @@ private fun reportError(codeFragment: KtCodeFragment, position: SourcePosition?,
         )
 
         LOG.error(
-            "Cannot evaluate a code fragment of type " + codeFragment::class.java + ": " + message.decapitalize(),
+            "Cannot evaluate a code fragment of type " + codeFragment::class.java + ": " + message.replaceFirstChar(Char::lowercaseChar),
             throwable,
             mergeAttachments(*attachments)
         )

idea/jvm-debugger/jvm-debugger-util/src/org/jetbrains/kotlin/idea/debugger/DebuggerUtils.kt

@@ -143,7 +143,7 @@ object DebuggerUtils {
     }
 
     fun isKotlinSourceFile(fileName: String): Boolean {
-        val extension = FileUtilRt.getExtension(fileName).toLowerCase()
+        val extension = FileUtilRt.getExtension(fileName).lowercase()
         return extension in KotlinFileTypeFactoryUtils.KOTLIN_EXTENSIONS
     }
 

idea/kotlin-gradle-tooling/src/org/jetbrains/kotlin/kapt/idea/KaptModelBuilderService.kt

@@ -72,7 +72,7 @@ class KaptModelBuilderService : AbstractKotlinGradleModelBuilder() {
                 }
 
                 val sourceSetName = compileTask.getSourceSetName()
-                val isTest = sourceSetName.toLowerCase().endsWith("test")
+                val isTest = sourceSetName.lowercase().endsWith("test")
 
                 val kaptGeneratedSourcesDir = getKaptDirectory("getKaptGeneratedSourcesDir", project, sourceSetName)
                 val kaptGeneratedClassesDir = getKaptDirectory("getKaptGeneratedClassesDir", project, sourceSetName)
@@ -91,7 +91,7 @@ class KaptModelBuilderService : AbstractKotlinGradleModelBuilder() {
                     val compilations = target.compilations ?: continue
                     for (compilation in compilations) {
                         val compileTask = compilation.getCompileKotlinTaskName(project) ?: continue
-                        val moduleName = target.name + compilation.name.capitalize()
+                        val moduleName = target.name + compilation.name.replaceFirstChar(Char::uppercaseChar)
                         handleCompileTask(moduleName, compileTask)
                     }
                 }

idea/kotlin-gradle-tooling/src/utils.kt

@@ -30,4 +30,4 @@ fun ClassLoader.loadClassOrNull(name: String): Class<*>? {
 }
 
 fun compilationFullName(simpleName: String, classifier: String?) =
-    if (classifier != null) classifier + simpleName.capitalize() else simpleName
+    if (classifier != null) classifier + simpleName.replaceFirstChar(Char::uppercaseChar) else simpleName

idea/performanceTests/test/org/jetbrains/kotlin/idea/perf/AbstractPerformanceCompletionHandlerTests.kt

@@ -43,7 +43,7 @@ abstract class AbstractPerformanceCompletionHandlerTests(
     protected open val statsPrefix = "completion"
 
     private fun stats(): Stats {
-        val suffix = "${defaultCompletionType.toString().toLowerCase()}${if (note.isNotEmpty()) "-$note" else ""}"
+        val suffix = "${defaultCompletionType.toString().lowercase()}${if (note.isNotEmpty()) "-$note" else ""}"
         return statsMap.computeIfAbsent(suffix) {
             Stats("$statsPrefix-$suffix")
         }

idea/src/org/jetbrains/kotlin/idea/inspections/SuspiciousCollectionReassignmentInspection.kt

@@ -65,7 +65,7 @@ class SuspiciousCollectionReassignmentInspection : AbstractKotlinInspection() {
                 else -> fixes.add(IntentionWrapper(ReplaceWithOrdinaryAssignmentIntention(), binaryExpression.containingKtFile))
             }
 
-            val typeText = leftDefaultType.toString().takeWhile { it != '<' }.toLowerCase()
+            val typeText = leftDefaultType.toString().takeWhile { it != '<' }.lowercase()
             val operationReference = binaryExpression.operationReference
             holder.registerProblem(
                 operationReference,

idea/src/org/jetbrains/kotlin/idea/intentions/ConvertFunctionTypeParameterToReceiverIntention.kt

@@ -183,7 +183,7 @@ class ConvertFunctionTypeParameterToReceiverIntention : SelfTargetingRangeIntent
                         if (callable !is PsiNamedElement) continue
 
                         if (!checkModifiable(callable)) {
-                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).capitalize()
+                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).replaceFirstChar(Char::uppercaseChar)
                             conflicts.putValue(callable, KotlinBundle.message("can.t.modify.0", renderedCallable))
                         }
 

idea/src/org/jetbrains/kotlin/idea/intentions/ConvertFunctionTypeReceiverToParameterIntention.kt

@@ -120,7 +120,7 @@ class ConvertFunctionTypeReceiverToParameterIntention : SelfTargetingRangeIntent
                         if (callable !is KtFunction) continue
 
                         if (!checkModifiable(callable)) {
-                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).capitalize()
+                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).replaceFirstChar(Char::uppercaseChar)
                             conflicts.putValue(callable, KotlinBundle.message("can.t.modify.0", renderedCallable))
                         }
 

idea/src/org/jetbrains/kotlin/idea/intentions/ConvertPropertyToFunctionIntention.kt

@@ -103,7 +103,7 @@ class ConvertPropertyToFunctionIntention : SelfTargetingIntention<KtProperty>(
                         if (callable !is PsiNamedElement) continue
 
                         if (!checkModifiable(callable)) {
-                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).capitalize()
+                            val renderedCallable = RefactoringUIUtil.getDescription(callable, true).replaceFirstChar(Char::uppercaseChar)
                             conflicts.putValue(callable, KotlinBundle.message("can.t.modify.0", renderedCallable))
                         }
 

idea/src/org/jetbrains/kotlin/idea/intentions/MoveMemberToCompanionObjectIntention.kt

@@ -281,7 +281,7 @@ class MoveMemberToCompanionObjectIntention : SelfTargetingRangeIntention<KtNamed
             return
         }
 
-        val description = RefactoringUIUtil.getDescription(element, false).capitalize()
+        val description = RefactoringUIUtil.getDescription(element, false).replaceFirstChar(Char::uppercaseChar)
 
         if (HierarchySearchRequest(element, element.useScope, false).searchOverriders().any()) {
             return CommonRefactoringUtil.showErrorHint(

idea/src/org/jetbrains/kotlin/idea/quickfix/AddArrayOfTypeFix.kt

@@ -23,7 +23,7 @@ class AddArrayOfTypeFix(expression: KtExpression, expectedType: KotlinType) : Ko
         "arrayOf"
     } else {
         val typeName = DescriptorRenderer.SHORT_NAMES_IN_TYPES.renderType(expectedType)
-        "${typeName.decapitalize(Locale.US)}Of"
+        "${typeName.replaceFirstChar(Char::lowercaseChar)}Of"
 
     }
 

idea/src/org/jetbrains/kotlin/idea/quickfix/createFromUsage/createClass/CreateClassFromUsageFix.kt

@@ -55,7 +55,7 @@ enum class ClassKind(@NonNls val keyword: String, @Nls val description: String)
     DEFAULT("", "") // Used as a placeholder and must be replaced with one of the kinds above
 }
 
-fun ClassKind.toIdeaClassKind() = IdeaClassKind { this@toIdeaClassKind.description.capitalize() }
+fun ClassKind.toIdeaClassKind() = IdeaClassKind { this@toIdeaClassKind.description.replaceFirstChar(Char::uppercaseChar) }
 
 val ClassKind.actionPriority: IntentionActionPriority
     get() = if (this == ANNOTATION_CLASS) IntentionActionPriority.LOW else IntentionActionPriority.NORMAL
@@ -185,7 +185,7 @@ open class CreateClassFromUsageFix<E : KtElement> protected constructor(
             val defaultPackageFqName = file.packageFqName
             val dialog = object : CreateKotlinClassDialog(
                 file.project,
-                KotlinBundle.message("create.0", ideaClassKind.description.capitalize()),
+                KotlinBundle.message("create.0", ideaClassKind.description.replaceFirstChar(Char::uppercaseChar)),
                 className,
                 defaultPackageFqName.asString(),
                 ideaClassKind,

idea/src/org/jetbrains/kotlin/idea/refactoring/changeSignature/KotlinChangeSignatureUsageProcessor.kt

@@ -789,7 +789,7 @@ class KotlinChangeSignatureUsageProcessor : ChangeSignatureUsageProcessor {
                     val prefix = if (declaration != null) RefactoringUIUtil.getDescription(declaration, true) else originalRef.text
                     result.putValue(
                         originalRef,
-                        KotlinBundle.message("text.0.will.no.longer.be.accessible.after.signature.change", prefix.capitalize())
+                        KotlinBundle.message("text.0.will.no.longer.be.accessible.after.signature.change", prefix.replaceFirstChar(Char::uppercaseChar))
                     )
                 }
             }

idea/src/org/jetbrains/kotlin/idea/refactoring/inline/AbstractKotlinInlineDialog.kt

@@ -52,7 +52,7 @@ abstract class AbstractKotlinInlineDialog(
 
     override fun getNameLabelText(): String {
         val occurrencesString = occurrencesString?.let { " - $it" } ?: ""
-        return "${kind.capitalize()} ${callable.nameAsSafeName} $occurrencesString"
+        return "${kind.replaceFirstChar(Char::uppercaseChar)} ${callable.nameAsSafeName} $occurrencesString"
     }
 
     private fun getInlineText(verb: String) = KotlinBundle.message(

idea/src/org/jetbrains/kotlin/idea/refactoring/introduce/introduceProperty/KotlinInplacePropertyIntroducer.kt

@@ -74,21 +74,22 @@ class KotlinInplacePropertyIntroducer(
         if (availableTargets.size > 1) {
             addPanelControl(
                 ControlWrapper {
-                    val propertyKindComboBox = with(JComboBox(availableTargets.map { it.targetName.capitalize() }.toTypedArray())) {
+                    val propertyKindComboBox =
-                        addPopupMenuListener(
+                        with(JComboBox(availableTargets.map { it.targetName.replaceFirstChar(Char::uppercaseChar) }.toTypedArray())) {
-                            object : PopupMenuListenerAdapter() {
+                            addPopupMenuListener(
-                                override fun popupMenuWillBecomeInvisible(e: PopupMenuEvent?) {
+                                object : PopupMenuListenerAdapter() {
-                                    ApplicationManager.getApplication().invokeLater {
+                                    override fun popupMenuWillBecomeInvisible(e: PopupMenuEvent?) {
-                                        currentTarget = availableTargets[selectedIndex]
+                                        ApplicationManager.getApplication().invokeLater {
+                                            currentTarget = availableTargets[selectedIndex]
+                                        }
                                     }
                                 }
-                            }
+                            )
-                        )
 
-                        selectedIndex = availableTargets.indexOf(currentTarget)
+                            selectedIndex = availableTargets.indexOf(currentTarget)
 
-                        this
+                            this
-                    }
+                        }
 
                     val propertyKindLabel = JLabel(KotlinBundle.message("label.text.introduce.as"))
                     propertyKindLabel.labelFor = propertyKindComboBox

idea/src/org/jetbrains/kotlin/idea/refactoring/kotlinRefactoringUtil.kt

@@ -233,7 +233,7 @@ fun reportDeclarationConflict(
     declaration: PsiElement,
     message: (renderedDeclaration: String) -> String
 ) {
-    conflicts.putValue(declaration, message(RefactoringUIUtil.getDescription(declaration, true).capitalize()))
+    conflicts.putValue(declaration, message(RefactoringUIUtil.getDescription(declaration, true).replaceFirstChar(Char::uppercaseChar)))
 }
 
 fun <T, E : PsiElement> getPsiElementPopup(

idea/src/org/jetbrains/kotlin/idea/refactoring/move/moveDeclarations/MoveFilesWithDeclarationsViewDescriptor.kt

@@ -27,7 +27,7 @@ internal class MoveFilesWithDeclarationsViewDescriptor(
                 "move.single.element.elements.header",
                 UsageViewUtil.getType(myElementsToMove[0]),
                 newParent.virtualFile.presentableUrl
-            ).capitalize()
+            ).replaceFirstChar(Char::uppercaseChar)
             myCodeReferencesText = KotlinBundle.message(
                 "text.references.in.code.to.0.1.and.its.declarations",
                 UsageViewUtil.getType(myElementsToMove[0]),

idea/src/org/jetbrains/kotlin/idea/refactoring/move/moveDeclarations/moveConflictUtils.kt

@@ -416,7 +416,7 @@ class MoveConflictChecker(
                     render(container),
                     render(referencedElement)
                 )
-                conflicts.putValue(element, message.capitalize())
+                conflicts.putValue(element, message.replaceFirstChar(Char::uppercaseChar))
             }
         }
     }
@@ -494,7 +494,7 @@ class MoveConflictChecker(
                                 render(declaration),
                                 render(target)
                             )
-                            conflicts.putValue(refExpr, message.capitalize())
+                            conflicts.putValue(refExpr, message.replaceFirstChar(Char::uppercaseChar))
                         }
                     }
             }
@@ -527,7 +527,7 @@ class MoveConflictChecker(
                         render(container),
                         render(memberToCheck)
                     )
-                    conflicts.putValue(element, message.capitalize())
+                    conflicts.putValue(element, message.replaceFirstChar(Char::uppercaseChar))
                 }
             }
         }
@@ -573,7 +573,8 @@ class MoveConflictChecker(
             val message = if (elementToMove == rootClass) {
                 KotlinBundle.message("text.sealed.class.0.must.be.moved.with.all.its.subclasses", rootClass.name.toString())
             } else {
-                val type = ElementDescriptionUtil.getElementDescription(elementToMove, UsageViewTypeLocation.INSTANCE).capitalize()
+                val type = ElementDescriptionUtil.getElementDescription(elementToMove, UsageViewTypeLocation.INSTANCE)
+                    .replaceFirstChar(Char::uppercaseChar)
                 KotlinBundle.message(
                     "text.0.1.must.be.moved.with.sealed.parent.class.and.all.its.subclasses",
                     type,

idea/src/org/jetbrains/kotlin/idea/refactoring/pullUp/pullUpConflictsUtils.kt

@@ -93,7 +93,7 @@ internal fun checkVisibilityInAbstractedMembers(
                 val memberText = memberDescriptor.renderForConflicts()
                 val targetText = targetDescriptor.renderForConflicts()
                 val message = KotlinBundle.message("text.0.uses.1.which.will.not.be.accessible.from.subclass", memberText, targetText)
-                conflicts.putValue(target, message.capitalize())
+                conflicts.putValue(target, message.replaceFirstChar(Char::uppercaseChar))
             }
         }
     }
@@ -148,7 +148,7 @@ private fun KotlinPullUpData.checkClashWithSuperDeclaration(
 
     if (member is KtParameter) {
         if (((targetClass as? KtClass)?.primaryConstructorParameters ?: emptyList()).any { it.name == member.name }) {
-            conflicts.putValue(member, message.capitalize())
+            conflicts.putValue(member, message.replaceFirstChar(Char::uppercaseChar))
         }
         return
     }
@@ -158,7 +158,7 @@ private fun KotlinPullUpData.checkClashWithSuperDeclaration(
     val clashingSuper = getClashingMemberInTargetClass(memberDescriptor) ?: return
     if (clashingSuper.modality == Modality.ABSTRACT) return
     if (clashingSuper.kind != CallableMemberDescriptor.Kind.DECLARATION) return
-    conflicts.putValue(member, message.capitalize())
+    conflicts.putValue(member, message.replaceFirstChar(Char::uppercaseChar))
 }
 
 private fun PsiClass.isSourceOrTarget(data: KotlinPullUpData): Boolean {
@@ -198,7 +198,7 @@ private fun KotlinPullUpData.checkAccidentalOverrides(
                         memberDescriptor.renderForConflicts(),
                         it.resolveToDescriptorWrapperAware(resolutionFacade).renderForConflicts()
                     )
-                    conflicts.putValue(clashingMember, message.capitalize())
+                    conflicts.putValue(clashingMember, message.replaceFirstChar(Char::uppercaseChar))
                 }
         }
     }
@@ -211,7 +211,7 @@ private fun KotlinPullUpData.checkInnerClassToInterface(
 ) {
     if (isInterfaceTarget && memberDescriptor is ClassDescriptor && memberDescriptor.isInner) {
         val message = KotlinBundle.message("text.inner.class.0.cannot.be.moved.to.intefrace", memberDescriptor.renderForConflicts())
-        conflicts.putValue(member, message.capitalize())
+        conflicts.putValue(member, message.replaceFirstChar(Char::uppercaseChar))
     }
 }
 
@@ -231,7 +231,7 @@ private fun KotlinPullUpData.checkVisibility(
                 memberDescriptor.renderForConflicts(),
                 targetDescriptor.renderForConflicts()
             )
-            conflicts.putValue(target, message.capitalize())
+            conflicts.putValue(target, message.replaceFirstChar(Char::uppercaseChar))
         }
     }
 

idea/src/org/jetbrains/kotlin/idea/refactoring/pushDown/KotlinPushDownHandler.kt

@@ -44,7 +44,8 @@ class KotlinPushDownHandler : AbstractPullPushMembersHandler(
 
     private fun reportFinalClassOrObject(project: Project, editor: Editor?, classOrObject: KtClassOrObject) {
         val message = RefactoringBundle.getCannotRefactorMessage(
-            KotlinBundle.message("text.class.0.is.final", RefactoringUIUtil.getDescription(classOrObject, false)).capitalize()
+            KotlinBundle.message("text.class.0.is.final", RefactoringUIUtil.getDescription(classOrObject, false))
+                .replaceFirstChar(Char::uppercaseChar)
         )
         CommonRefactoringUtil.showErrorHint(project, editor, message, PULL_MEMBERS_UP, HelpID.MEMBERS_PULL_UP)
     }

idea/src/org/jetbrains/kotlin/idea/refactoring/pushDown/KotlinPushDownProcessor.kt

@@ -100,7 +100,7 @@ class KotlinPushDownProcessor(
         val usages = refUsages.get() ?: UsageInfo.EMPTY_ARRAY
         if (usages.isEmpty()) {
             val message = KotlinBundle.message("text.0.have.no.inheritors.warning", context.sourceClassDescriptor.renderForConflicts())
-            val answer = Messages.showYesNoDialog(message.capitalize(), PUSH_MEMBERS_DOWN, Messages.getWarningIcon())
+            val answer = Messages.showYesNoDialog(message.replaceFirstChar(Char::uppercaseChar), PUSH_MEMBERS_DOWN, Messages.getWarningIcon())
             if (answer == Messages.NO) return false
         }
 

idea/src/org/jetbrains/kotlin/idea/refactoring/pushDown/pushDownConflictsUtils.kt

@@ -88,7 +88,7 @@ private fun checkConflicts(
             targetClassDescriptor.renderForConflicts(),
             context.sourceClassDescriptor.renderForConflicts()
         )
-        conflicts.putValue(targetClass, message.capitalize())
+        conflicts.putValue(targetClass, message.replaceFirstChar(Char::uppercaseChar))
     }
 
     for (member in membersToPush) {
@@ -146,7 +146,7 @@ private fun checkMemberClashing(
                         targetClassDescriptor.renderForConflicts(),
                         CommonRefactoringUtil.htmlEmphasize(member.name ?: "")
                     )
-                    conflicts.putValue(it, message.capitalize())
+                    conflicts.putValue(it, message.replaceFirstChar(Char::uppercaseChar))
                 }
         }
     }
@@ -217,7 +217,7 @@ private fun checkVisibility(
                 targetDescriptor.renderForConflicts(),
                 targetClassDescriptor.renderForConflicts()
             )
-            conflicts.putValue(target, message.capitalize())
+            conflicts.putValue(target, message.replaceFirstChar(Char::uppercaseChar))
         }
     }
 

idea/src/org/jetbrains/kotlin/idea/refactoring/rename/RenameKotlinPropertyProcessor.kt

@@ -114,7 +114,7 @@ class RenameKotlinPropertyProcessor : RenameKotlinPsiProcessor() {
         result: MutableList<UsageInfo>
     ) {
         fun reportAccidentalOverride(candidate: PsiNamedElement) {
-            val what = UsageViewUtil.getType(declaration).capitalize()
+            val what = UsageViewUtil.getType(declaration).replaceFirstChar(Char::uppercaseChar)
             val withWhat = candidate.renderDescription()
             val where = candidate.representativeContainer()?.renderDescription() ?: return
             val message = KotlinBundle.message("text.0.will.clash.with.existing.1.in.2", what, withWhat, where)

idea/src/org/jetbrains/kotlin/idea/refactoring/rename/renameConflictUtils.kt

@@ -166,7 +166,7 @@ internal fun checkRedeclarations(
         if (overloadChecker != null && overloadChecker.isOverloadable(descriptor, candidateDescriptor)) continue
         val what = candidate.renderDescription()
         val where = candidate.representativeContainer()?.renderDescription() ?: continue
-        val message = KotlinBundle.message("text.0.already.declared.in.1", what, where).capitalize()
+        val message = KotlinBundle.message("text.0.already.declared.in.1", what, where).replaceFirstChar(Char::uppercaseChar)
         result += BasicUnresolvableCollisionUsageInfo(candidate, candidate, message)
     }
 }
@@ -197,7 +197,7 @@ fun reportShadowing(
         "text.0.will.be.shadowed.by.1",
         declaration.renderDescription(),
         candidate.renderDescription()
-    ).capitalize()
+    ).replaceFirstChar(Char::uppercaseChar)
     result += BasicUnresolvableCollisionUsageInfo(refElement, elementToBindUsageInfoTo, message)
 }
 

idea/tests/org/jetbrains/kotlin/idea/configuration/AbstractConfigureKotlinInTempDirTest.kt

@@ -20,7 +20,7 @@ abstract class AbstractConfigureKotlinInTempDirTest : AbstractConfigureKotlinTes
         FileUtil.copyDir(File(projectRoot), tempDir)
 
         val kotlinRuntime = File(tempDir, "lib/kotlin-stdlib.jar")
-        if (getTestName(true).toLowerCase().contains("latestruntime") && kotlinRuntime.exists()) {
+        if (getTestName(true).lowercase().contains("latestruntime") && kotlinRuntime.exists()) {
             ForTestCompileRuntime.runtimeJarForTests().copyTo(kotlinRuntime, overwrite = true)
         }
 

idea/tests/org/jetbrains/kotlin/idea/quickfix/AbstractQuickFixTest.kt

@@ -67,7 +67,7 @@ abstract class AbstractQuickFixTest : KotlinLightCodeInsightFixtureTestCase(), Q
     }
 
     override fun getProjectDescriptor(): LightProjectDescriptor =
-        if ("createfromusage" in testDataPath.toLowerCase()) {
+        if ("createfromusage" in testDataPath.lowercase()) {
             KotlinWithJdkAndRuntimeLightProjectDescriptor.INSTANCE
         } else {
             super.getProjectDescriptor()

idea/tests/org/jetbrains/kotlin/idea/resolve/AbstractResolveModeComparisonTest.kt

@@ -139,7 +139,7 @@ abstract class AbstractResolveModeComparisonTest : KotlinLightCodeInsightFixture
             val modes = mark.modes
             val modesRepresentation =
                 if (modes.size == RESOLVE_MODES.size) "All"
-                else modes.joinToString { it.toString().toLowerCase().capitalize() }
+                else modes.joinToString { it.toString().lowercase().replaceFirstChar(Char::uppercaseChar) }
             "/*$modesRepresentation (${mark.ordinal})*/"
         }
         is Mark.NotAnalyzed -> {

idea/tests/org/jetbrains/kotlin/idea/script/AbstractScriptConfigurationTest.kt

@@ -131,7 +131,7 @@ abstract class AbstractScriptConfigurationTest : KotlinCompletionTestCase() {
         }
 
         // If script is inside module
-        if (module != null && mainScriptFile.parentFile.name.toLowerCase().contains("module")) {
+        if (module != null && mainScriptFile.parentFile.name.lowercase().contains("module")) {
             module.addDependency(
                 projectLibrary(
                     "script-runtime",

j2k/src/org/jetbrains/kotlin/j2k/ast/Expressions.kt

@@ -295,7 +295,7 @@ class ClassLiteralExpression(val type: Type): Expression() {
 fun createArrayInitializerExpression(arrayType: ArrayType, initializers: List<Expression>, needExplicitType: Boolean = true) : MethodCallExpression {
     val elementType = arrayType.elementType
     val createArrayFunction = when {
-        elementType is PrimitiveType -> (elementType.toNotNullType().canonicalCode() + "ArrayOf").decapitalize(Locale.US)
+        elementType is PrimitiveType -> (elementType.toNotNullType().canonicalCode() + "ArrayOf").replaceFirstChar(Char::lowercaseChar)
         needExplicitType -> "arrayOf<" + arrayType.elementType.canonicalCode() + ">"
         else -> "arrayOf"
     }

jps-plugin/jps-tests/test/org/jetbrains/kotlin/jps/build/AbstractIncrementalJpsTest.kt

@@ -276,7 +276,7 @@ abstract class AbstractIncrementalJpsTest(
         }
 
         if (!makeOverallResult.makeFailed) {
-            if (checkDumpsCaseInsensitively && rebuildResult.mappingsDump?.toLowerCase() == makeOverallResult.mappingsDump?.toLowerCase()) {
+            if (checkDumpsCaseInsensitively && rebuildResult.mappingsDump?.lowercase() == makeOverallResult.mappingsDump?.lowercase()) {
                 // do nothing
             } else {
                 TestCase.assertEquals(rebuildResult.mappingsDump, makeOverallResult.mappingsDump)

jps-plugin/jps-tests/test/org/jetbrains/kotlin/jps/build/dependeciestxt/MppJpsIncTestsGenerator.kt

@@ -29,7 +29,7 @@ fun actualizeMppJpsIncTestCaseDirs(rootDir: String, dir: String) {
 }
 
 class MppJpsIncTestsGenerator(val txt: ModulesTxt, val testCaseDirProvider: (TestCase) -> File) {
-    val ModulesTxt.Module.capitalName get() = name.capitalize()
+    val ModulesTxt.Module.capitalName get() = name.replaceFirstChar(Char::uppercaseChar)
 
     val testCases: List<TestCase>
 
@@ -308,7 +308,7 @@ class MppJpsIncTestsGenerator(val txt: ModulesTxt, val testCaseDirProvider: (Tes
         protected fun serviceKtFile(module: ModulesTxt.Module, fileNameSuffix: String = ""): File {
             val suffix =
                 if (module.isCommonModule) "${module.serviceName}Header"
-                else "${module.name.capitalize()}${module.contentsSettings.serviceNameSuffix}Impl"
+                else "${module.name.replaceFirstChar(Char::uppercaseChar)}${module.contentsSettings.serviceNameSuffix}Impl"
 
             return File(dir, "${module.indexedName}_service$suffix.kt$fileNameSuffix")
         }

js/js.dce/src/org/jetbrains/kotlin/js/dce/util.kt

@@ -47,7 +47,7 @@ fun Context.isObjectFunction(function: JsExpression, functionName: String): Bool
 fun Context.isKotlinFunction(function: JsExpression, name: String): Boolean {
     if (function !is JsNameRef || function.ident != name) return false
     val receiver = (function.qualifier as? JsNameRef)?.name ?: return false
-    return receiver in nodes && receiver.ident.toLowerCase(Locale.US) == "kotlin"
+    return receiver in nodes && receiver.ident.lowercase() == "kotlin"
 }
 
 fun isSpecialFunction(expr: JsExpression, specialFunction: SpecialFunction): Boolean =

js/js.frontend/src/org/jetbrains/kotlin/js/config/ErrorTolerancePolicy.kt

@@ -18,7 +18,7 @@ enum class ErrorTolerancePolicy(val allowSyntaxErrors: Boolean, val allowSemanti
         val DEFAULT = NONE
 
         fun resolvePolicy(key: String): ErrorTolerancePolicy {
-            return when (key.toUpperCase(Locale.US)) {
+            return when (key.uppercase()) {
                 "NONE" -> NONE
                 "SEMANTIC" -> SEMANTIC
                 "SYNTAX", "ALL" -> ALL

js/js.tests/test/org/jetbrains/kotlin/generators/tests/generateTestDataForReservedWords.kt

@@ -311,7 +311,10 @@ private class TestDataBuilder {
                             .replace(KEYWORD_MARKER, keywordWithEscapeIfNeed)
 
 
-                    val fileName = "${suite.name.decapitalize()}${case.name.capitalize()}${keyword.capitalize()}.kt"
+                    val decapitalizedSuiteName = suite.name.replaceFirstChar(Char::lowercaseChar)
+                    val capitalizedCaseName = case.name.replaceFirstChar(Char::uppercaseChar)
+                    val capitalizedKeyword = keyword.replaceFirstChar(Char::uppercaseChar)
+                    val fileName = "$decapitalizedSuiteName$capitalizedCaseName$capitalizedKeyword.kt"
 
                     val testDataFile = File(testDataDirPath + "/" + fileName)
 

js/js.translator/src/org/jetbrains/kotlin/js/translate/intrinsic/functions/factories/ArrayFIF.kt

@@ -109,7 +109,7 @@ object ArrayFIF : CompositeFIF() {
     }
 
     private val PrimitiveType.lowerCaseName
-        get() = typeName.asString().toLowerCase(Locale.US)
+        get() = typeName.asString().lowercase()
 
     fun getTag(descriptor: CallableDescriptor, config: JsConfig): String? {
         if (descriptor !is ConstructorDescriptor) return null